1. Field of the Invention
This invention relates to a method of and apparatus for casting metal to form metal slabs. More particularly, the invention relates to continuous casting methods and apparatus in which the metal is cast in a casting cavity formed between spaced confronting casting surfaces that are advanced in a casting direction between an inlet for and an outlet from the casting cavity.
2. Background Art
Elongated relatively thin metal slabs (sometimes also referred to as cast strips or bands) may be produced by continuous casting techniques in equipment such as twin-belt casters, rotating block casters, twin-roll casters, and the like. Metals that have moderate or relatively low melting temperatures, e.g. aluminum, magnesium, zinc, and alloys having these elements as principal ingredients, are particularly suitable for this kind of casting, but other metals may also be cast in such equipment on occasion. Heat is withdrawn from the metal in the casting cavity by and through the casting surfaces so that the metal cools and produces a solid slab having a thickness similar to the spacing between the casting surfaces. Side dams are usually provided between the casting surfaces at their extreme lateral edges to prevent loss of metal and to define the side edges of the casting cavity. A molten metal injector or launder is used to continuously introduce molten metal into the casting cavity through the inlet and the solidified slab is continuously withdrawn from the casting cavity through the outlet by the motion of the casting surfaces. The casting surfaces are continuously recirculated externally of the casting cavity from the outlet to the inlet so that they are continuously available for use.
The casting surfaces are generally actively cooled so that they are capable of withdrawing heat from the metal in the casting cavity. This can be done, for example, by applying a coolant, e.g. a cooling liquid or possibly a gas, to recirculating elements on which the casting surfaces are formed, which elements normally have good heat conduction properties so that heat passes through them from the metal to the coolant. In the case of twin-belt casters, for example, cooling liquid (usually water containing appropriate additives) is applied to the rear surfaces of recirculating casting belts in the regions where the belts confront each other to form the casting cavity so that heat is conducted from the casting cavity through the casting surfaces and the belts and is removed by the coolant. Examples of twin-belt casters of this kind are described in U.S. Pat. No. 4,061,178 which issued to Sivilotti et al. on Dec. 6, 1977; U.S. Pat. No. 4,193,440 which issued to Thorburn et al. on Mar. 18, 1980; and U.S. patent publication No. 2010/0307713 which published on Dec. 9, 2010 in the names of Ito et al. The disclosures of these patents are specifically incorporated herein by this reference.
When operating apparatus of this kind, it is usual to maintain an even cooling of the casting surfaces at all locations along the casting cavity in the direction of casting and to keep the casting surfaces in firm contact with the molten or solidifying metal at all such locations in order to maintain the ability of the casting surfaces to withdraw heat from the metal undergoing casting. Since the metal may contract slightly as it cools and solidifies on its passage through the casting cavity, the casting surfaces may be made to converge slightly towards each other in the direction from the inlet to the outlet so that firm contact with the metal is maintained throughout the casting cavity. However, when metal is cast in this way, the rate at which heat is withdrawn from the metal (i.e. the heat flux through the casting surfaces) is initially high because of the large difference in temperature between the molten metal undergoing casting and the cooled casting surfaces and by good conformal contact between the molten metal and the casting surfaces. As casting proceeds further, the outer surfaces of the embryonic metal slab are cooled more quickly than the central parts of the metal slab since temperature equalization within the metal takes time. As the outer slab surfaces cool, heat flux through the casting surfaces declines because of the reduction in the temperature differential between the casting surfaces and the adjacent metal. Eventually, the outer surfaces of the metal begin to solidify, even though the central parts may still be molten. It is necessary to ensure that the casting cavity has a sufficient length (distance between the inlet and the outlet in the casting direction) to allow for sufficient heat withdrawal before the cast slab is discharged through the outlet. In practice, the casting cavity must be of such a length that the exit temperature of the slab (generally as measured at the outer surface) is low enough that the slab can be subjected to further handling and processing without deformation or damage. Of course, the necessary length of the casting cavity is also linked to the rate of throughput of the metal in that, for a given metal or alloy, a slower rate of metal throughput will allow more time for heat withdrawal and will therefore allow the casting cavity to be made shorter than would be the case for a higher rate of metal throughput. Twin-roll casters, in particular, employ a very short casting cavity that is formed essentially by the nip between the rolls.
The need for slow rates of metal throughput and/or long casting cavities results in higher equipment and production costs than would be the case if rates could be increased and/or casting cavities shortened. Longer casting times and cavity lengths may also require greater amounts of coolant to be employed. There is therefore a desire to design and operate casting apparatus of this kind in such a way that casting rates can be further increased and/or casting cavities shortened.